Habitual marijuana (search) use increased among U.S. adults over the past decade, particularly among young minorities and baby boomers, government figures show.

The prevalence of marijuana abuse or dependence climbed from 1.2 percent of adults in 1991-92 to 1.5 percent in 2001-02, or an estimated 3 million adults 18 and over.

That represents an increase of 800,000 people, according to data from two nationally representative surveys that each queried more than 40,000 adults.

Among 18- to 29-year-olds, the rate or abuse or dependence remained stable among whites but surged by about 220 percent among black men and women, to 4.5 percent of that population, and by almost 150 percent among Hispanic men, to 4.7 percent.

Among all adults ages 45 to 64, the rate increased by 355 percent, to about 0.4 percent of that population.

The report, published in Wednesday's Journal of the American Medical Association, was led by Dr. Wilson Compton of the National Institute on Drug Abuse (search), who said the rise in dependence was probably due at least partly to increases in the potency of pot over the past decade.

Also, the figures may indicate that baby boomers "bring their bad habits with them into old age," he said.

The researchers said adults were considered marijuana abusers if repeated use of the drug hurt their ability to function at work, in school or in social situations, or created drug-related legal problems.

Drug users were considered dependent if they experienced increased tolerance of marijuana, used it compulsively and continued using it despite drug-related physical or psychological problems.

Overall use of the drug -- that is, casual use and habitual use -- remained stable at around 4 percent of adults.

"This study suggests that we need to develop ways to monitor the continued rise in marijuana abuse and dependence and strengthen existing prevention and intervention efforts," said Dr. Nora Volkow, the institute's director. Programs that target young black and Hispanic adults are particularly needed, she said.

Increases in dependence among young minorities may reflect their growing assimilation into sectors of white society where marijuana use is more accepted, Compton said.

Researchers from the National Institute on Alcohol Abuse and Alcoholism (search) contributed to the report.